Dental bridge alloy

Abstract

A highly ductile cobalt-chromium-nickel dental alloy suited for crown and bridge applications which require deformation by hand burnishing in the mouth of a patient, has a controlled high stacking fault energy and a ductility of about 20 percent elongation or more, a yield strength of about 35,000 psi or less and low work hardening characteristics and is free of hardening precipitates resulting from the inclusion of significant amounts of carbon, boron, molybdenum, titanium, aluminum, and tungsten in the alloy. Preferably, the nickel-cobalt ratio in the alloy base ranges from a low of about 1:3, by weight, to prevent the normal transformation of cobalt from a ductile face centered cubic lattice to a hexagonal close pack structure upon cooling to room temperature to a high of 2:1 since higher concentrations of nickel produce CoNi 3 . The alloy base preferably includes 20 percent chromium to provide the necessary corrosion resistance and an additional amount of 2 percent nickel is required in the alloy base to offset the adverse effects of chromium on the ductility of the alloy. A small quantity of niobium or tantalum, both of which have substantial solubility in the alloy base, are added to form nuclei of crystallization for the alloy to promote fineness of crystal size and uniformity of physical properties from melt-to-melt. Niobium and tantalum also counteract the adverse effect of chromium on the stacking fault energy of the alloy with 1 percent of niobium, or approximately 2 percent of tantalum, counteracting the adverse effect of 1.25 percent chromium so that the nickel used in the alloy base can be reduced according to the amount of niobium or tantalum added to the alloy.

I claim: 
     
       1. A highly ductile cobalt-chromium-nickel dental alloy having a ductility of about 20 percent or more elongation, a yield strength of about 35,000 or less and low work hardening characteristics suited for crown and bridge applications requiring deformation by hand burnishing in the mouth of a patient, said alloy having no more than 0.02 percent carbon and being essentially free of boron, molybdenum, titanium, aluminum and tungsten to prevent the formation of hardening precipitates thereof in the alloy and having an alloy base consisting essentially of, by weight, about 10 to 60 percent cobalt, 17 to 24 percent chromium, and 20 to 75 percent nickel as the essential major alloying elements, and an element of the group consisting of tantalum and niobium alloyed therewith to promote a high rate of crystallization and provide uniformity and fineness of crystal size, said alloying element being present in an amount of up to about 4 percent tantalum or an equivalent atomic weight of niobium. 
     
     
       2. An alloy according to claim 1 wherein nickel is present, by weight, in the alloy base in an amount about twice the amount of chromium plus an additional amount in the ratio to cobalt of between about 1 to 3 and 2 to 1. 
     
     
       3. An alloy according to claim 2 wherein the alloying element is present in the amount of 2 percent tantalum or the equivalent atomic weight of niobium. 
     
     
       4. A dental casting made of the alloy of claim 1. 
     
     
       5. An alloy according to claim 2 wherein the alloy base consists essentially of about 20 percent chromium, 50 to 67 percent nickel, and 13 to 30 percent cobalt, by weight, and the alloying element is present in the amount of up to about 2 percent tantalum or the equivalent atomic weight of niobium. 
     
     
       6. A dental casting made of the alloy of claim 5. 
     
     
       7. A dental casting made of the alloy of claim 4. 
     
     
       8. The method of making a cobalt-nickel-chromium alloy suited for use as a gold alloy substitute for dental crown and bridge applications requiring hand burnishing in the mouth of a patient and having a controlled high stacking fault energy, a ductility of about 20 percent or more elongation, a yield strength of about 35,000 psi or less, and low work hardening characteristics, said alloy having an alloy base consisting essentially of about 20 to 75 percent nickel, 17 to 24 percent chromium, and 10 to 60 percent cobalt, by weight, comprising the steps of providing nickel in an amount of about twice as much as the chronium present, of providing the remainder of the alloy base by adding cobalt and an additional amount of nickel in the ratio of nickel to cobalt of between about 1:3 and 2:1, by weight and the step of stabilizing the uniformity and fineness of the crystal structure of the alloy by adding an element selected from the group consisting of niobium and tantalum in an amount of up to about 4 percent tantalum, by weight, or the equivalent atomic weight of niobium to promote a high rate of crystallization and provide fineness and uniformity of grain size in the alloy.